PurePontiacKid Posted March 19, 2012 Share Posted March 19, 2012 Hi, I was wondering if someone could shed some light on my little problem I'm having. I just swapped out my stock EFI in my 79 280ZX for some 4 screw SUs I rebuilt and installed SM needles, and now when i m at WOT in any gear the motor just hits a wall and sounds like it has a limiter at 5k rpm. Will free-rev just fine. Also, if I go only about 3/4 throttle, the car will pull to 6k without any problems. Its definitely going lean, doesn't feel like an ignition issue. Have checked ignition timing, 15* initial, 35* all in. No vacuum leaks. Checked coil with known good unit. Car has Aeromotive carbureted street fuel pump (90gph). Mallory fuel pressure regulator (set at 5psi, have tried higher, not lower yet) and K&N filters. MSD Blaster coil and stock ignition ottherwise. Other than the WOT issue, the car runs perfect. Rock solid idle, no smoke, etc. Anybody have any insight? Quote Link to comment Share on other sites More sharing options...
BlueStag Posted March 19, 2012 Share Posted March 19, 2012 Mallory fuel pressure regulator (set at 5psi, have tried higher, not lower yet) and K&N filters. Anybody have any insight? I can tell you very little except to say that SU's never need to see more than 5lbs. At least that has been my long understanding. I have always understood that they want 3lbs at the most, really. Do remember that there is a difference between pressure and rate of flow, but I am sure that you know that, based on the detail in your question. The engine was performing fine with the FI at higher rpm under load? Have you tried running the carbs on the rich side? Just to see how they perform? It is by no means impossible that the needles that you have are wrong for the engine you have. They could be lean at full throttle but correct thru the first 3/4 of the throttle. Unhappily, I don't think anything but a dyno is going to clear up that question for you. Quote Link to comment Share on other sites More sharing options...
madkaw Posted March 20, 2012 Share Posted March 20, 2012 You could try tweaking the floats up richer, you maybe running lean in WOT. Quote Link to comment Share on other sites More sharing options...
PurePontiacKid Posted March 20, 2012 Author Share Posted March 20, 2012 Yeah that's one of the only things I can think of, too... Did more testing today, and free-revving real hard (6k+ rpm) and fuel would shoot out of the front carb's float vent. Also noticed quite a bit of vapor shooting out of the same carb... Tried changing needle/seat/float from my other set with no change. Quote Link to comment Share on other sites More sharing options...
19762802+2 Posted March 20, 2012 Share Posted March 20, 2012 what carb fluid are you running? Quote Link to comment Share on other sites More sharing options...
PurePontiacKid Posted March 20, 2012 Author Share Posted March 20, 2012 0w-20 synthetic engine oil Quote Link to comment Share on other sites More sharing options...
Cannonball89 Posted March 20, 2012 Share Posted March 20, 2012 Fuel shooting out of the carb vent would indicate that the float isn't sealing the float bowl properly, or that the fuel pressure is rising too high at high RPM and overwhelming the float. Could be that your car is bogging from going way too rich. Are you using your FPR as a rising rate regulator or just as a fixed pressure regulator? Are you using the stock carburetor fuel rail (the one that has the little pin hole in the return portion that acted as the factory FPR) or some other kind of fuel rail? Pictures here would help. And just to make sure it's not an ignition problem, what impedance is your coil? If it is 3 ohms, then you cannot have a ballast resistor. Quote Link to comment Share on other sites More sharing options...
PurePontiacKid Posted March 20, 2012 Author Share Posted March 20, 2012 I don't believe its rich because there is no smoke coming from the tailpipe and the spark plugs are white/tan. I'm not running a fuel rail, hose straight to the carbs and the regulator has the return going straight to the stock return line in the car. It is a fixed pressure regulator. The ignition coil is the same from when I had EFI, which ran just fine to 6k with no issues (other than not being able to breathe well because of the stock EFI and intake lol) Quote Link to comment Share on other sites More sharing options...
Leon Posted March 20, 2012 Share Posted March 20, 2012 I'm not sure on how you determined that it is "definitely going lean". Do you have a wideband? If not, then my first thought is that you're flooding the engine with fuel. The SM needles and 5 psi of fuel pressure are the likely causes. The fuel shooting out of the float bowl is a clear sign of this. Make sure your floats are set correctly as well, some SUs had different float heights front-rear depending on year. Quote Link to comment Share on other sites More sharing options...
PurePontiacKid Posted March 20, 2012 Author Share Posted March 20, 2012 White spark plugs, no black smoke, no smell of unburned fuel coming from the exhaust typically leads me to believe that a car is running lean. If it was running rich enough to bog the motor, id think the plugs would be black, and thered be a cloud of black smoke, along with a definite smell of unburned fuel from the exhaust... Quote Link to comment Share on other sites More sharing options...
Leon Posted March 20, 2012 Share Posted March 20, 2012 White spark plugs, no black smoke, no smell of unburned fuel coming from the exhaust typically leads me to believe that a car is running lean. If it was running rich enough to bog the motor, id think the plugs would be black, and thered be a cloud of black smoke, along with a definite smell of unburned fuel from the exhaust... True, but only if the car is running VERY rich all the time. If you only get soggy at a certain condition, your plugs, exhaust, etc. will appear perfectly fine. My point is, you could be lean or rich, don't assume one way or another without hard evidence. Check all possibilities. My triples are jetted too rich right now. There is no black smoke and plugs look great, but I cruise at about 11-11.5 AFR. Sometimes I even drop below 11 AFR but I wouldn't know it without a wideband. WOT mixture is a bit leaner, maybe 12 AFR and there are no issues with bogging at that AFR. Quote Link to comment Share on other sites More sharing options...
Cannonball89 Posted March 20, 2012 Share Posted March 20, 2012 So far the only empirical evidence we have is that it is shooting fuel out of the float bowls when free revving the engine. If I were you I would fix whatever is causing that before you hydrolock your motor at WOT. Quote Link to comment Share on other sites More sharing options...
PurePontiacKid Posted March 21, 2012 Author Share Posted March 21, 2012 To clear things up, it only shot out of the front carb. Swapped needle/seat/float from another set of SUs I have and it doesn't do that anymore, but it is still having the same issue. I have tried running the car with the fuel pump disconnected (to see if it was too much pressure, can't turn it down less than 4.5psi) and that didn't change anything. Then tried turning the mixture adjusters all the way lean, and now the car pulls to just about 6k and hits the wall there, so apparently I was wrong in thinking it was a leaning out issue when in reality it is running too rich. Quote Link to comment Share on other sites More sharing options...
PurePontiacKid Posted March 22, 2012 Author Share Posted March 22, 2012 Car is running great now! Swapped my 3-screw bodies, dashpots and nozzles (swapped everything else over to them) and that solved the issue! Not sure wtf was goin on with the 4-screws, but I honestly don't care right now, enjoying the amazing intake noise I get now! Haha Quote Link to comment Share on other sites More sharing options...
BluDestiny Posted March 22, 2012 Share Posted March 22, 2012 oics Quote Link to comment Share on other sites More sharing options...
PurePontiacKid Posted March 22, 2012 Author Share Posted March 22, 2012 As soon as I get these presentable. Didn't get a chance to make the tops look pretty lol Quote Link to comment Share on other sites More sharing options...
Leon Posted March 23, 2012 Share Posted March 23, 2012 Then tried turning the mixture adjusters all the way lean, and now the car pulls to just about 6k and hits the wall there, so apparently I was wrong in thinking it was a leaning out issue when in reality it is running too rich. Bingo... Quote Link to comment Share on other sites More sharing options...
Leon Posted March 23, 2012 Share Posted March 23, 2012 Car is running great now! Swapped my 3-screw bodies, dashpots and nozzles (swapped everything else over to them) and that solved the issue! Not sure wtf was goin on with the 4-screws, but I honestly don't care right now, enjoying the amazing intake noise I get now! Haha My bet is on the nozzles sticking in the down position, allowing too much fuel past the needle. Remove the nozzles, clean them up and they should be okay, unless something is seriously wrong. Quote Link to comment Share on other sites More sharing options...
PurePontiacKid Posted March 23, 2012 Author Share Posted March 23, 2012 I tried doing that before I swapped carbs. They were only sticking a tiny bit and only by hand, polished them up to where they didn't stick at all, and didn't change a thing.. Quote Link to comment Share on other sites More sharing options...
Leon Posted March 23, 2012 Share Posted March 23, 2012 I tried doing that before I swapped carbs. They were only sticking a tiny bit and only by hand, polished them up to where they didn't stick at all, and didn't change a thing.. Then, the next thing to do would be to verify float level using clear tubing attached to the bowl outlet as a gauge. Quote Link to comment Share on other sites More sharing options...
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